1. Field of the Invention
This invention relates to planetary gearing, clutches and brakes for an automatic transmission used in motor vehicles. The invention pertains, more particularly, to a transaxle having two planetary gear units that produce an overdrive speed, direct drive, and two underdrive speeds.
2. Description of the Prior Art
Various arrangements of clutches, brakes and one-way clutches are used in the prior art to control operation of dual interconnected planetary gear units to produce forward speed ratios and a reverse drive ratio in an automatic transmission.
U.S. Pat. No. 4,418,585 has clutches and brakes arranged so that a gear ratio change from the lowest speed to the second speed is made nonsynchronously, i.e., by transferring torque from an overrunning coupling to a friction clutch. In that transmission, a gear shift from the second speed to the third speed requires disengagement of the brake band and application of a clutch. A gear ratio change from the third speed to the fourth or overdrive speed also requires disengagement of a brake band and engagement of a friction clutch. In the operation of the transmission, none of the gear shifts require synchronous disengagement of a clutch and engagement of another clutch. Therefore, timing problems the engagement and release of the clutch brake control servos are eliminated.
The transmission according to the '585 patent requires time for disengagement of a high speed ratio clutch and application of a friction brake in order to produce the ratio change from the third forward speed to the fourth overdrive speed.
U.S. Pat. No. 4,368,649 describes a four-speed transaxle that overcomes this difficulty. In the transaxle of the '649 patent the gear shift from the third speed to the fourth speed results by applying a single friction brake in addition to the other friction elements engaged during the third speed ratio. A gear shift from the first speed to the second forward speed results merely by engaging a second friction clutch while a companion friction clutch remains applied. In this way, a ratio change from the first ratio and from the third ratio in the forward driving speed range results merely by engaging or disengaging a single friction element, either a clutch or a brake, thereby greatly simplifying control of the clutches and eliminating potential for harsh or abrupt gear shift changes.
U.S. Pat. No. 4,509,389 describes a further improvement that eliminates a latent difficulty in control of the transmission of the '649 patent that makes calibration of the two-three upshift difficult. The sun gear is not connected to a friction clutch cylinder but is connected instead to the inner race of an associated one-way clutch. The inner races of each one-way coupling are connected to a common member, which operates as a torque delivery element for the input sun gear of the planetary gear. The maximum speed of the friction clutch cylinders is equal to the speed of the driven sprocket of a chain mechanism connecting the output of a torque converter to the input shaft of the transmission.
U.S. Pat. No. 4,086,827 describes a four speed transmission in which a single one-way clutch is located in series between an input friction clutch and a gear member of a planetary gear set. The one-way clutch permits the gear member to overrun the input during an overdrive ratio so that an upshift from the third speed to the fourth speed results without a synchronous release of the input friction clutch. To produce a downshift from the fourth speed to the second or third speeds, the input increases to the speed of the gear member by engagement of the one-way clutch when a friction brake or another friction clutch is released.